Over the last several years, the PI and his colleagues have shown that motoneuron pools map onto muscles with a rostrocaudal positional bias. Detailed studies from their lab revealed that this topographic map is detectable in embryonic muscles upon first contact between nerve and muscle, and is partially restored after denervation. They have developed and important model of synaptic competition during reinnervation, where they can predict with 95% accuracy the survivor between two competing nerve terminals. They have also developed an in vitro model to identify muscle membrane-bound labels that may be responsible for the positional bias. They have bound selective growth of embryonic spinal cord neurites on membranes derived from embryonic rostral or caudal muscles. They have also found selective growth of embryonic spinal cord neurites on membrane derived from transgenic muscle cell lines bearing a heritable memory for rostrocaudal position. They propose a series of experiments to extend these observations. First, using the serratus anterior (SA) muscle as a model of synaptic competition during reinnervation, they will use activity-dependent fluorescent dyes to differentially label nerve terminals in dually- innervated end-plates receiving inputs from both C6 and C7 ventral roots. Second, building upon their observations that embryonic caudal spinal cord neurites grow selectively on caudal muscle membranes, they will determine the period of expression of a label during development, and whether a continuous gradient of the label is expressed along the rostrocaudal axis. Third, they will experimentally manipulate the spinal cord neurites or substrate to test the behavior of neurites on gradients of membranes or carpets of only one membrane type. They will also ask whether sensory neurites respond to a positional label expressed by muscle membranes. Fourth, they will ask whether the label is expressed in rostral or caudal membranes (or both). Whether recognition of the label occurs between rat and chick tissues, and whether it leads to growth cone collapse. They will also extend their observations of selective growth of neurites on membranes derived from transgenic muscle cells. Finally, they will begin a series of experiments in an attempt to isolate and characterize the positional label such as developing and testing antibodies against the label, and determining its molecular mature, as a prelude to its isolation. Results of these studies will provide unique insight into how neurites in the peripheral nervous system recognize and synapse with their positionally matched partners. We will also learn whether positional labels in the neuromuscular system are part of a general scheme for encoding position in the nervous system.